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This article is about specialization of function between the left and right hemispheres of the brain. For specialization of brain function generally, see Functional specialization (brain).

Broad generalizations are often made in popular psychology about one side or the other having characteristic labels, such as "logical" for the left side or "creative" for the right. These labels are not supported by studies on lateralization, as lateralization does not add specialized usage from either hemisphere. [1] Both hemispheres contribute to both kinds of processes.[2] and experimental evidence provides little support for correlating the structural differences between the sides with such broadly-defined functional differences.[3]

The extent of any modularity, or specialization of brain function by area, remains under investigation. If a specific region of the brain, or even an entire hemisphere, is either injured or destroyed, its functions can sometimes be assumed by a neighboring region in the ipsilateral hemisphere or a corresponding region in the contralateral hemisphere, depending upon the area damaged and the patient's age.[4] When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

Brain function lateralization is evident in the phenomena of right- or left-handedness[5] and of right or left ear preference,[6] but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions.[7] Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.[8]

Additionally, although some functions are lateralized, these are only a tendency. The trend across many individuals may also vary significantly as to how any specific function is implemented. The areas of exploration of this causal or effectual difference of a particular brain function include its gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.[9]

In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.[14][15] Functions such as the processing of visual and audiological stimuli, spatial manipulation, facial perception, and artistic ability seem to be functions of the right hemisphere.

One of the first indications of brain function lateralization resulted from the research of French physician Pierre Paul Broca, in 1861. His research involved the male patient nicknamed "Tan", who suffered a speech deficit (aphasia); "tan" was one of the few words he could articulate, hence his nickname. In Tan's autopsy, Broca determined he had a syphilitic lesion in the left cerebral hemisphere. This left frontal lobe brain area (Broca's area) is an important speech production region. The motor aspects of speech production deficits caused by damage to Broca’s area are known as Expressive aphasia. In clinical assessment of this aphasia, it is noted that the patient cannot clearly articulate the language being employed.

German physician Karl Wernicke continued in the vein of Broca's research by studying language deficits unlike expressive aphasia. Wernicke noted that not every deficit was in speech production; some were linguistic. He found that damage to the left posterior, superior temporalgyrus (Wernicke's area) caused language comprehension deficits rather than speech production deficits, a syndrome known as Receptive aphasia.

These seminal works on hemispheric specialization were done on patients and/or postmortem brains, raising questions about the potential impact of pathology on the research findings. New methods permit the in vivo comparison of the hemispheres in healthy subjects. Particularly, magnetic resonance imaging (MRI) and positron emission tomography (PET) are important because of their high spatial resolution and ability to image subcortical brain structures.

In the past it was believed that there were lateralization differences between men and women, and that these differences led to hemisphere specialization of cognitive function. [22] This was an inaccurate understanding based on studies done of localized areas of each hemisphere. Male brains are no more lateralized than female brains. Neither men or women are biased by specialized usage from either hemisphere of their brain through lateralization. [1]

Language functions such as grammar, vocabulary and literal meaning[23][21] are typically lateralized to the left hemisphere, especially in right handed individuals.[21] While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral, or even right lateralized in approximately 50% of left-handers.[24] In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.[25][26]

The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show a right hemisphere superiority.[24] Numerical estimation, comparison and online calculation depend on bilateral parietal regions[16][17] while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing.[16][17]Dyscalculia is a neurological syndrome associated with damage to the left temporo-parietal junction.[12] This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.[13]

Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in "processing negative emotions, pessimistic thoughts and unconstructive thinking styles", as well as vigilance, arousal and self-reflection, and a relatively hypoactive left hemisphere, "specifically involved in processing pleasurable experiences" and "relatively more involved in decision-making processes".[27] Additionally, "left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern."[28] The delusional misidentification syndromes, reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.[29][30] There is evidence[31] that the right hemisphere is more involved in processing novel situations, while the left hemisphere is most involved when routine or well rehearsed processing is called for.

Broca's area and Wernicke's area are linked by a white matter fiber tract, the arcuate fasciculus[dubious — see talk page]. This axonal tract allows the neurons in the two areas to work together in creating vocal language. In more than 95% of right-handed men, and more than 90% of right-handed women, the left hemisphere is dominant in certain aspects of language and speech processing. In left-handed people, the incidence of left-hemisphere language dominance has been reported as 73%[32] and 61%,[7] suggesting left handed people tend to be less lateralized than right-handed people. In general, however, neuroimaging methods such as functional magnetic resonance imaging and magnetoencephalography show involvement of both hemispheres in many aspects of language processing, and the "dominance" of one hemisphere just refers to more brain activation relative to the other hemisphere (or better performance by that hemisphere on psycholinguistic tasks such as dichotic listening); it is not the case that language is "localized" in any one hemisphere.

Research by Michael Gazzaniga and Roger Wolcott Sperry in the 1960s on split-brain patients led to an even greater understanding of functional laterality. Split-brain patients are patients who have undergone corpus callosotomy (usually as a treatment for severe epilepsy), a severing of a large part of the corpus callosum. The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other. This led to many interesting behavioral phenomena that allowed Gazzaniga and Sperry to study the contributions of each hemisphere to various cognitive and perceptual processes. One of their main findings was that the right hemisphere was capable of rudimentary language processing, but often has no lexical or grammatical abilities.[33] Eran Zaidel, however, also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.

For example: Patients with brain damage from surgery, stroke or infection sometimes develop a syndrome in which they can feel sensations in their hand, but they don't feel responsible for nor able to control its movements. In patients with a corpus callosotomy, alien hand syndrome most often manifests as uncontrolled but purposeful movements of the nondominant hand.[citation needed]

Terence Hines states that the research on brain lateralization is valid as a research program, though commercial promoters have applied it to promote subjects and products far outside the implications of the research.[34] For example, the implications of the research have no bearing on psychological interventions such as EMDR and neurolinguistic programming,[35] brain training equipment, or management training.[36]

Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds and mammals with the left hemisphere being specialized to categorize information and control everyday, routine behavior, with the right hemisphere responsible for responses to novel events and behavior in emergencies including the expression of intense emotions. An example of a routine left hemisphere behavior is feeding behavior whereas as a right hemisphere is escape from predators and attacks from conspecifics.[37]

The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere.[38] In one experiment, baby chicks were lateralized before hatching by exposing their eggs to light.[39] These chicks were set to a task of picking out food from a bed of pebbles. Neither the lateralized, nor the non-lateralized chicks had a problem with this task, but the lateralized chicks only used the eye on the side of which they were lateralized to pick up the pebbles. When presented with a second task of watching for a cutout of a predatory hawk, the discrepancy between lateralized and non-lateralized chicks became evident. Lateralized chicks could pick food out of the pebbles with one eye and one half of the brain[40] while using the other eye and other half of their brain to monitor the skies for predators.[41] Not only could non-lateralized chicks not complete the two tasks simultaneously, but their performance of the single task deteriorated. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.[38]